1. Field of the Invention
The present invention relates to optical metrology, and more particularly to improving the optical metrology precision and procedures applying noise reduction to the data.
2. Description of the Related Art
Optical metrology involves directing an incident beam at a feature on a wafer, measuring the resulting diffraction signal, and analyzing the measured diffraction signal to determine various characteristics of the feature. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
With increased requirement on measurement precision, the measured diffraction signal may be relatively weak and not provide enough signal-to-noise (S/N) ratio. For example, the measured diffraction signal may include noise related to the hardware used to obtain the measured diffraction signal and to the feature being measured. The noise in the measured diffraction signal may decrease the precision and accuracy of the optical metrology process.